Polytetrafluoroethylene (PTFE) fine powder has been used for many years as a wire insulator. However, despite the desirable properties of high molecular weight PTFE, namely low dielectric constant and low dissipation factor, other polymers have been used in forming insulation for wires, especially in large diameter, high frequency coaxial transmission cable because of the processing limitations of PTFE.
Polytetrafluoroethylene (PTFE) fine powder is a type of PTFE that is made by aqueous dispersion polymerization, followed by coagulation of the dispersion and drying of the resultant coagulated solids to obtain the fine powder. Because the PTFE fine powder does not flow in the melt condition sufficiently to enable melt processing, the powder has been fabricated into articles and coated onto wire by an extrusion method which does not require melt flow. This extrusion method is known as paste extrusion and is described for example in U.S. Pat. No. 2,685,707. In paste extrusion, a paste extrusion composition is formed by mixing PTFE fine powder with an organic lubricant which has a viscosity of at least 0.45 centipoise at 25° C. and is liquid under the conditions of subsequent extrusion. The PTFE soaks up the lubricant, resulting in a dry, pressure coalescing paste extrusion composition that is also referred to as lubricated PTFE fine powder. During paste extrusion which is typically performed at a temperature of 20 to 60° C., the lubricated fine powder is forced through a die to form a lubricated green extrudate. The lubricated green extrudate is then heated, usually at a temperature of 100 to 250° C., to make volatile and drive off the lubricant from the extrudate. In most cases, the dried extrudate is heated to a temperature close to or above the melting point of the PTFE, typically between 327° C. and 500° C., to sinter the PTFE.
Cables capable of transmitting high frequency electromagnetic radiation are especially useful for communications in the radio bandwidth in areas where structures may inhibit signal transmission, such as in or around buildings, tunnels, or garages. Likewise, such cables also have use in unobstructed areas, but where there is a need for precisely controlled signal levels that must be distributed over distances without the interference of other nearby signals.
High frequency transmission cable requires insulation with the lowest possible dielectric constant ∈T and the lowest possible dissipation factor tan δ to obtain the desired attenuation characteristics. As disclosed in U.S. Pat. No. 5,922,155, a dielectric constant of less than about 1.8 is desirable. Insulation material such as fluoropolymers and polyethylene have dielectric constants close to about 2. As taught in U.S. Pat. Nos. 3,771,934 and 5,922,155, air cells can be introduced into insulation made of melt-flowable polymers to reduce the dielectric constant.
PTFE has the lowest loss (dissipation factor) of most commonly used polymers. So while other polymers such as polyethylene or melt-flowable fluoropolymers, e.g., PFA [TFE/perfluoro(alkyl vinyl ether) copolymer] and FEP (TFE/hexafluoropropylene copolymer), are easier to form by using melt extrusion techniques, their loss is not as low as PTFE. For example, at a frequency of 1 MHz, the dissipation factor of PTFE is more than two times lower than PFA and more than three times lower than polyethylene or FEP. Nonetheless, PTFE has been difficult to form into large diameter insulators for wire because of difficulties in drying the green extrudate to remove the lubricant from thick structures of paste extruded PTFE fine powder. Large diameter PTFE insulators have also been expensive because of the quantity of PTFE fine powder required.
Another significant requirement for high frequency communication cable is noncombustibility that fluoropolymers can provide but structures of polyethylene, polypropylene or polyvinyl chloride cannot. PTFE has the highest stability to heat among the fluoropolymers.
An insulated wire of paste extruded PTFE with lower dielectric constant, with lower dissipation factor and which requires a smaller quantity of PTFE fine powder for its manufacture would be highly desirable for use in high frequency cables.